In the field of polymers, in particular when the polymers are brittle, it is common practice to add additives in order to confer impact strength on these polymers (these additives commonly being denoted under the terminology of “impact modifier”).
Thus, these additives can be provided in the form of elastomeric particles introduced into the polymer matrix. For example, MBS additives, MBS denoting a particulate copolymer comprising a crosslinked core based essentially on butadiene and styrene and a polymethyl methacrylate shell, can be used to reinforce the impact strengths of various polymer matrices, such as rigid PVC matrices, polycarbonate matrices or polyester matrices, it being possible for the applications targeted to be construction materials, such as window sections, or packaging materials for products in everyday use (such as computer or telephone casings).
There may exist a number of applications where the initial optical transparency of the polymer has to be maintained after addition of said additives.
One solution which can be envisaged for maintaining the optical transparency of the polymer is to alter the chemical composition of the additive by adjusting the composition so that the refractive index of the additive is close to that of the polymer matrix in which the additive is intended to be incorporated. Thus, before the incorporation of the additive in the matrix, it is necessary to carry out refractive index measurements on said synthesized additive in order to be able to determine if it is suitable for the matrix for which it is intended. The aim is to avoid or greatly restrict the phenomenon of light scattering which would result in the opaqueness of the initially transparent matrix. Mention may be made, as example of additive capable of being incorporated in a transparent matrix, such as a polyvinyl chloride matrix, without harming the initial transparency of the latter, of a core-shell additive of the MBS type, the shell of which is composed of units resulting from the polymerization of methyl methacrylate and the core of which is composed of units resulting from the polymerization of 1,4-butadiene and styrene.
However, the fact of having to alter the composition of the additives in order to maintain the transparency properties of the recipient polymer matrix sometimes contributes to reducing the effectiveness of the additive as regards its ability to reinforce the impact strength of the matrices for which it is intended. Thus, in the case of the MBS core-shell additives for polyvinyl chloride matrices, the presence of styrene units in an increasing content in the core reduces the elastomeric nature of the additive (in particular by increasing the glass transition temperature and the elastic modulus) and consequently its effectiveness.
For specific polymer matrices, such as polycarbonate matrices, it is also impossible to adjust the refractive index of an additive so that the latter protects the optical transparency properties of the host matrix. However, some authors have provided block copolymers (such as described in WO 2003/062293). In this case, the transparency of the host polymer matrix depends entirely on the nanostructuring which occurs during the processing of the blend composed of the constituent polymer of the matrix and of the block copolymer. This nanostructuring can only occur under certain conditions, such as:                the compatibility between the matrix and one of the blocks of the additive used;        the blending conditions, such as the shear rate;        the kinetics of cooling after the blending.        
In other words, the final optical transparency of a matrix reinforced by the additive will never be certain of being maintained as it depends on complex mechanisms of separation of phases. There thus exists a true need for a process for the preparation of polymer additives, which additives can be used in polymer matrices without harming their transparency properties, which is simple to implement, in particular so as to be able to be carried out in an industrial environment.